Ranking is a central topic in the field of search mainly because the performance of a search system can be evaluated by the accuracy of its ranking results. Traditionally, a ranking model is defined as a function of a query and a document, which represents the relevance of the document with respect to the query. In search, when given a query, the ranking model assigns a score to each of the documents retrieved with the query and produces the ranked list of documents sorted by the scores. The ranking model is typically a local model in the sense that the creation and utilization of the model only needs the involvement of a single document.
As search evolves, more and more useful information for ranking becomes available. This includes the content information of documents as well as the relation information between documents. The relations can be hyperlinks between documents, similarities between documents, etc. Ideally, the ranking model would be a function of the query and all the retrieved documents with the query. That is to say, ranking should be conducted on the basis of the contents of documents as well as the relations between documents.